The present invention relates to a mobile communication apparatus which allows the coexistence of a cellular automobile telephone system and a micro cellular system, and more particularly to a mobile communication apparatus which provides the systems with an increased capacity for personal stations.
A mobile communication apparatus which allows the coexistence of a cellular automobile telephone system and a micro cellular system is disclosed in JP-A-4-200031, and its investigation for commercialization is still under progress.
The mobile communication apparatus now in a practical phase employs an analog communication form and a frequency division multiplex access mode which assigns radio channels at different frequencies to respective subscribers requesting a call.
As shown in FIG. 1, this apparatus accesses to the cellular automobile telephone system when a personal station (PS) 16 is transported by a car or the like which is running at a high speed, or to the micro cellular system when a personal station is carried by a person who is moving in a crowded area such as a building, an amusement quarter, or the like at a low speed. A system to be accessed is manually or automatically switched.
In the cellular automobile telephone system, a public switched telephone network (PSTN) 11 is connected to a personal station 16 through a mobile telephone switching office (MTSO) 12 and a base station (BS) 13 placed under the command of the mobile telephone switching office 12. Generally, a predetermined number of radio channels are previously assigned to the base station 13, so that the mobile telephone switching office 12 selects an unused radio channel from the assigned radio channels prior to communications to establish a communication line between the base station 13 and the personal station 16.
In the cellular automobile telephone system, a service area is divided into small zones corresponding to respective base stations. Between base stations 13 which are located too remotely to interfere with each other, the same frequency is repeatedly used to cover a wider area with a high frequency utilization efficiency.
On the other hand, in the micro cellular system, the public switched telephone network 11 is connected to a personal station 16 through a control unit (CU) 14 and a fixed station (FS) 15 placed under the command of the control unit 14. The control unit 14 utilizes a cellular scanner (CS) 19 to search for radio channels which are being used by neighboring base stations of the cellular automobile telephone system, and assigns one of the unused radio channels to a communication channel for use between the fixed station 15 and the personal station 16 to establish a communication line therebetween.
In FIG. 1, an area covered by the base station 13 of the cellular automobile telephone system is indicated by a solid line circle 17, and an area covered by the fixed station 15 of the micro cellular system is indicated by a broken line circle 18.
In this manner, the conventional mobile communication apparatus improves the frequency utilization efficiency by sharing the same frequency band with the cellular automobile telephone system and the micro cellular system.
The conventional mobile communication system, however, implies problems as follows:
(a) Since the conventional mobile communication system employs an analog communication form, the frequency utilization efficiency cannot be highly improved, so that the number of available personal stations will not be significantly increased. PA0 (b) If the micro cellular system is installed at an open place such as the upper stories of a building, radio waves from the cellular automobile telephone system will easily reach the micro cellular system even from remote locations. Therefore, it is quite difficult to find a radio channel which is free from interference with these radio waves.